As will be apparent from the following commonly owned U.S. Pat. Nos. 3,811,761, 3,890,841, 3,956,936, 3,964,314, 3,966,500 and 4,278,828 (see also German patent document DE-GM No. 71 11 837 corresponding to U.S. Pat. No. 3,964,314), temperature measuring instruments having a resistive element forming a sensor are known and are widely used for a variety of purposes in high temperature applications.
Such temperature measuring instruments are known as noise thermometers and utilize the fact that temperature is associated with variations in electrical parameters. These systems, more specifically, use the noise voltage produced in a conductor or resistive element by thermal agitation of electrically charged particles therein.
The thermal noise power is proportional to the resistance value, is proportional to the absolute temperature and, of course, is proportional in the frequency band width over which the noise is measured. The thermal noise is also known as Johnson noise.
All metals have the noise generation properties mentioned above, i.e. produce a noise voltage which is proportional to temperature. For the most part, platinum has been used heretofore in temperature sensors operating under the noise voltage principle.
Temperature sensors of the type described have the advantage that the output is usually discriminated from other changes and is not falsified by variations which are not proportional to temperature.
In the aforementioned patents, the sensing element is generally a thin strand of a pure metal or a metal alloy generally with a cross section corresponding to a circular area with a diameter between 5 microns and 50 microns and usually with a thickness in this range. The metals used include pure metals such as tungsten, tantalum, molybdenum, niobium, titanium, zirconium and platinum. Alloys of chromium, nickel and iron, preferably with these metals, are also described as being effective and the patents describe ceramic supports for the comparatively thin strands.
Noise thermometers are particularly suitable for measuring temperatures above 1150.degree. C. and thus the most advantageous group of metals and alloys are those of platinum, rhodium, tungsten and tantalum, i.e. metals which are refractory to temperatures of 1150.degree. C. and above.
As experience with such metals has progressed, it has been found that they have relatively high temperature coefficients of electrical resistance. As a consequence, relatively large values of the measured parameter V.sup.2, i.e. the mean square noise voltage, when the measuring temperature during the process is not sufficiently constant. One must thus tolerate an imprecise measurement.
The relatively high temperature coefficient of electrical resistance of these materials also creates difficulties with respect to the matching of the measuring resistance and the conductors which are to be joined thereto. In fact, a matching of this type is possible only for a comparatively small temperature measurement range.
Because of the relatively small specific resistance of such materials even at comparatively high temperatures, they had to be fabricated heretofore with very small diameters, usually below 0.2 mm or applied as relatively thin layers to a carrier. As a consesquence, it was difficult to provide the metals in resistance units of the requisite resistance value generally greater than 5 ohms, and simultaneously to make the sensor sufficiently small to permit it to be used as a point-like measuring sensor.
As the diameter of the wire was reduced, stability and mechanical strength factors entered into the design and created fabrication problems and problems with use because of mechanical sensitivity.
Furthermore, the refractory metals and their alloys can only be prepared at comparatively high cost in protective gas atmospheres or in vacuum.
It is known, from U.S. Pat. No. 2,710,899, to utilize oxide ceramics such as Al.sub.2 O.sub.3, CaO, ZrO.sub.2, BeO, ThO.sub.2, or MgO as noise thermometer resistance elements. These materials also have high temperature coefficients of resistivity so that the problems discussed above in connection with such coefficients remain. Further, they are also characterized by the aforedescribed problem of matching the measuring resistor and conductors running thereto.
Furthermore, the specific resistances of these oxide ceramics may also be too high at elevated temperatures to permit resistance elements with a resistivity of about 10 ohms to be fabricated as is preferred.
It is also a disadvantage of these oxide ceramics that they constitute ion conductors whose resistance at high temperatures may vary upon the application of a voltage, thereby creating problems in determining the measuring resistance R.sub.M. There is also some question as to whether ionic conductors in the frequency range of up to 300 kHz, common for noise thermometers, have a white frequency spectrum as is necessary for the usual comparison methods.